The present invention relates to high tension friction driven servo controlled linear motion systems, sometimes referred to as capstan drives, and more particularly to a method and apparatus for joining a metal drive band to the driven element using energy absorbing materials to absorb band vibration energy. Precision linear motion systems using servo controlled position or velocity loops, or both, provide linear motion in either a continuous or start-stop manner. The velocity and position of the driven element can be critical for only one direction of travel, in which case the driven element can be slewed back to a start position before the next motion, or the velocity and position of the driven element can be critical for both directions of travel.
A band drive system, without feedback to the control loop of driven element position, presents many problems because the band must never slip relative to the drive pulley nor can any stretching or vibration of the band be allowed which would cause motion disturbances to be transmitted to the driven element. In such an open loop system, where the input drive angular position, as determined by the drive's rotary encoder, is the only feedback of the linear position of the driven element, the designer must ensure that the drive torque is directly transmitted through the drive pulley to the band, and from the band to the driven element, without inducing errors in linear position and or velocity of the driven element.
Various methods of band mounting have been employed including rigid mounts in which no attempt is made to dampen unwanted motion in the band. For rigid mounting, problems occur when servo induced torque ripples at or near a resonant frequency of the band cause micro displacements of the band to be transferred to the driven element. These band vibration modes may be axial or transaxial, each causing different motion disturbance profiles to be transmitted to the driven element. The band vibration modes ultimately cause position or velocity errors at the driven element which are undetected by the servo motion control loop. Vibration energy from sources other than the servo drive system cause also cause the band to vibrate at its own natural frequency with the same result.
A prior art method for damping vibration energy in band drive systems is to wrap or coat the drive pulley with an energy absorbing layer. This method of wrapping the drive pulley is not acceptable in high pressure drive systems because with a compliant material wrapped around the pulley, the mean radius of the pulley becomes a function of band tension and absolute position and velocity errors of the driven element occur when the drive pulley radius varies.
It is accordingly, a general object of the present invention to provide a drive band-to-driven element joining method and apparatus that dampens unwanted energy in the band such that absolute position of the driven element is not a function of band tension.
It is another object of the invention to provide for energy absorption while still allowing the band to be maintained at a constant and high enough tension to prevent slippage between the band and the drive pulley.
It is a still another object of the invention to provide band vibration absorption over a wide enough range of vibration frequency to absorb the many band natural vibration frequencies associated with the changing length of unsupported band as the driven element moves from end to end of its travel.
It is a feature of the invention that the energy damping can be achieved through pre-compressed elements.
It is another feature of the invention that the final compression of the pre-compressed elements is accomplished by tensioning the drive band to its operating tension.